Typically, an RTA has a smaller build deck beneath/inside a tank. E-liquid is fed using vacuum. The benefits are long periods of vaping without muss, and some would say increased flavor because if the small build chamber. Downsides are the small build deck (harder to build), the potential for leaks, fragile glass...
On the other hand, RDAs generally have larger, easier to access decks, and, as liquid is applied in small quantities every couple pulls, the possibility of large leaks is reduced. Of course, that means stopping every couple pulls to add more E-liquid.
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